1. Field of the Invention
The present invention relates to a fuel injection valve for injecting fuel into an internal combustion engine.
2. Description of Related Art
In a fuel injection valve shown in U.S. Pat. No. 6,420,817B1, a valve closing operation is performed on a nozzle needle (i.e., the nozzle needle closes a nozzle hole) when a piezo stack is charged with electric charge to have a high voltage, and a valve opening operation is performed on the nozzle needle when the electric charge is discharged and thereby the piezo stack has a low voltage. In such a case, while an internal combustion engine is in operation, a period during which the piezo stack is at a high voltage is longer than a period during which the piezo stack is at a low voltage. Thus, the above fuel injection valve is not desirable in respect of reliability (e.g., durability) of the piezo stack.
Accordingly, a fuel injection valve, which is configured such that the valve opening operation is performed on the nozzle needle while the piezo stack is at a high voltage and the valve closing operation is performed on the nozzle needle while the piezo stack is at a low voltage, is proposed in, for example, JP-T-2007-500304 (corresponding to US2007/0152084A1).
As shown in FIG. 9, in the fuel injection valve of JP-T-2007-500304, when a piezo stack 900 has a high voltage and is thereby extended, a first transfer piston 910 is driven, and accordingly a pressure of fuel in an oil-tight chamber 920 rises. A pressure of fuel in the oil-tight chamber 920 is applied to an annular surface 941 of a second transfer piston 940 formed integrally with a nozzle needle 930, and as a result, the nozzle needle 930 opens a nozzle hole.
However, in the fuel injection valve of JP-T-2007-500304, there are three sliding portion clearances between a high pressure fuel passages 950, through which high pressure fuel flows, and the oil-tight chambers 920. Consequently, when the fuel injection valve is in the valve opening operation, an amount of fuel, which leaks out of the oil-tight chamber 920 to the high pressure fuel passage 950 through the above clearances, increases. In addition, the three sliding portion clearances are a clearance between the first transfer piston 910 and the second transfer piston 940, a clearance between the first transfer piston 910 and a sleeve 960, and a clearance between the nozzle needle 930 and a guide part 970.
When the amount of fuel leaking out of the oil-tight chamber 920 to the high pressure fuel passage 950 increases, drive transmission efficiency is decreased. In the present specification, the decrease in the drive transmission efficiency means a decrease in an actual displacement enlargement factor (=a lift amount of a nozzle needle/a stretch amount of a piezo actuator).
When the amount of fuel leaking out of the oil-tight chamber 920 to the high pressure fuel passage 950 increases, the nozzle lift decreases and thus a spray state deteriorates in a late phase of the injection due to a pressure drop of the oil-tight chamber 920, in the case of a long fuel injection period.
Furthermore, it is difficult to make large an area of the annular surface 941 of the second transfer piston 940. Therefore, a differential pressure between the oil-tight chamber 920 and the high pressure fuel passage 950 needs to be made larger, in order that the nozzle needle 930 opens the nozzle hole.
As the differential pressure between the oil-tight chamber 920 and the high pressure fuel passage 950 becomes larger, the amount of fuel leaking out of the oil-tight chamber 920 to the high pressure fuel passage 950 increases. As a result, problems such as the decrease in the drive transmission efficiency and the deterioration of the spray state become more noticeable.
Additionally, in order to make large the differential pressure between the oil-tight chamber 920 and the high pressure fuel passage 950, an extension amount of the piezo stack 900 needs to be increased for the fuel pressure of the oil-tight chamber 920 to be high enough. For this reason, charging energy supplied to the piezo stack 900 increases.